Sequentially charged nanocrystal light emitting device

ABSTRACT

A light emitting device including a transistor structure formed on a semiconductor substrate. The transistor structure having a source region, a drain region, a channel region between the source and drain regions, and a gate oxide on the channel region. The light emitting device including a plurality of nanocrystals embedded in the gate oxide, and a gate contact made of semitransparent or transparent material formed on the gate oxide. The nanocrystals are adapted to be first charged with first type charge carriers, and then provided second type charge carriers, such that the first and second type charge carriers form excitons used to emit light.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 60/560,167 entitled “Sequentially ChargedNanocrystal Light Emitting Device” filed Apr. 7, 2004, the entirecontent of which has been incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has certain rights in this invention pursuant toMURI Grant No. AFOSR #FA9550-04-1-0434 awarded by the Air Force Officeof Scientific Research.

FIELD OF THE INVENTION

This application relates to light emitting devices, and moreparticularly to light emitting devices comprising nanocrystals.

BACKGROUND

In conventional light emitting diodes (LEDs), a direct current isapplied through the device to create excitons in its active region,which in turn are emitted as light. However, when the direct current isapplied to the bulk semiconductor material of the LEDs, many of thecharge carriers that make up the applied current do not form excitons inthe active region of the device. Hence, much of the applied energy iswasted.

Because of the availability and maturity of silicon microelectronicstechnologies, it would be beneficial to use silicon for the fabricationof optoelectronic devices. Light emission in conventional bulksilicon-based devices, however, is constrained in wavelength to infraredemission, and is inefficient because of the indirect band gap ofsilicon. Due to its poor light emitting properties, silicon is typicallynot used to fabricate light emitting devices.

Therefore, it is desirable to provide a silicon or other semiconductorbased light emitting device which is very efficient, such that onlyminimal energy is wasted in constructing the electron-hole pairs thatgenerate the necessary excitons.

SUMMARY

In an exemplary embodiment of the present invention, a light emittingdevice including a transistor structure formed on a semiconductorsubstrate is provided. The transistor structure has a source region, adrain region, a channel region between the source and drain regions, anda gate oxide on the channel region. The light emitting device includes aplurality of nanocrystals embedded in the gate oxide, and a gate contactmade of semitransparent or transparent material formed on the gateoxide. The nanocrystals are adapted to be charged with first type chargecarriers, and then provided second type charge carriers, such that thefirst and second type charge carriers form excitons which can then beused to emit light.

In another exemplary embodiment according to the present invention, amethod of emitting light from a light emitting device is provided. Insuch an embodiment, the light emitting device includes a transistorstructure formed on a semiconductor substrate and having a sourceregion, a drain region, a channel region between the source and drainregions, a gate oxide on the channel region, and a plurality ofnanocrystals embedded in the gate oxide. The method includes injectingfirst type charge carriers into the nanocrystals; and after injectingthe first type charge carriers, injecting second type charge carriersinto the nanocrystals, such that the first and second type chargecarriers form excitons that can be used to emit light.

In yet another exemplary embodiment according to the present invention,a light emitting device is provided. The light emitting device includesa gate oxide formed on a semiconductor substrate, a floating gate arrayof nanocrystals embedded in the gate oxide, and a transparent orsemi-transparent gate contact formed on the gate oxide. The nanocrystalsare adapted to be first charged with first type charge carriers, andthen provided second type charge carriers, such that the first andsecond type charge carriers form excitons that can be used to emitlight.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be more readilycomprehended in view of the discussion herein and accompanying drawings.

FIG. 1 is a conceptualized schematic view of a transistor structure thatcan operate as a light emitting device in exemplary embodiments of thepresent invention;

FIG. 2A is a schematic view of a light emitting device according to anexemplary embodiment of the present invention, illustrating a method ofcharging nanocrystals with electrons through Fowler-Nordheim tunneling;

FIG. 2B is a schematic view of the light emitting device according tothe exemplary embodiment of the present invention, illustrating a methodof providing holes to the negatively charged nanocrystals via Coulombfield-enhanced Fowler-Nordheim tunneling;

FIG. 2C is a schematic view of the light emitting device according tothe exemplary embodiment of the present invention, illustrating theradiative recombination of holes and electrons;

FIGS. 3A-3C illustrate the process of radiative recombination of holesand electrons where the holes are first charged to the nanocrystals;

FIGS. 4A and 4B illustrate a non-radiative exciton recombination ofholes and electrons via transfer of energy to a secondary emitter, and asubsequent light emission from a secondary emitter;

FIG. 5 is a graph representing a comparison between photoluminescence(PL) and electroluminescence (EL) emission spectra measured from theexemplary embodiment of the present invention;

FIG. 6 is a graph that illustrates a relationship between the gatevoltage and the electroluminescence for the exemplary embodiment of thepresent invention; and

FIGS. 7 and 8, respectively, illustrate a variation ofelectroluminescence intensity with respect to driving gate frequency andgate voltage for the exemplary embodiment of the present invention.

DETAILED DESCRIPTION

In exemplary embodiments according to the present invention, a new typeof light emitting device based on the unique luminescence properties ofnanocrystals or quantum dots, is provided. When silicon is grown in theform of tiny nanocrystals or quantum dots, the light emitting propertiesof silicon improve as a consequence of confinement effects, which shieldcharge carriers (i.e., carriers) from defects and enhance lightemission. However, because of potential barriers that form on theirperiphery, finding an efficient method for injecting electrons and holesinto nanocrystals, which is necessary to stimulate the emission oflight, has been difficult. In addition, the nanocrystals or quantum dotsmust be electrically isolated from their environment, otherwise, theconfinement potential that gives them their unique properties is notprovided.

In the past, nanocrystal based light emitting devices were generallybased on conventional device geometries and conventional designs similarto that of a p-n diode LED, such as, for example, P-N junction LEDsfabricated using silicon or gallium arsenide (GaAs). As such,conventional light emitting devices based on semiconductor nanocrystalshave a design similar to that used for a bulk material light emittingdevice.

In exemplary embodiments of the present invention, a novel scheme forthe charge injection is provided. Instead of having a diode structurewhere the nanocrystals are in the middle of the junction, which wouldthen be driven by DC current, the light emitting device according toexemplary embodiments of the present invention has a structure similarto a field effect transistor, e.g., a metal-oxide-semiconductorfield-effect transistor (MOSFET). Hence, the light emitting device canbe said to have a transistor structure.

In more detail, the fabrication of the light emitting device is based ona conventional floating gate memory technology. In such a fabricationmethod, the nanocrystals are embedded in the gate oxide to form afloating gate. The charge carriers are then injected into thenanocrystals sequentially (e.g., holes first then electrons, or viceversa) from the channel of the device using an alternating current (AC)electric excitation. In other words, electrons and holes tunnelsequentially from the channel under the influence of an alternatingvoltage applied to the gate of the transistor structure of the lightemitting device. In this manner, the nanocrystals are charged in anAC-like manner such that photons are generated in bursts.

Because very little energy is wasted in constructing the electron-holepairs the light emitting device of the present invention is extremelyefficient. This can be contrasted with conventional LEDs, in which manyof the charge carriers that make up the applied current do not formexcitons in the active region of the device.

As previously discussed, during operation of the light emitting deviceof the present invention, initially first type charge carriers arecharged to the nanocrystals, and then second type charge carriers areprovided to the nanocrystals such that they together form excitons. Thefirst and second type charge carriers may be negative and positivecharge carriers (i.e., electrons and holes), respectively, or viceversa. This was an unexpected result because those skilled in the artwould have expected this scheme to simply pull in charge carriers of onetype and then immediately inject charge carriers of the other typewithout a significant formation of excitons through their interaction.

Another advantage of an LED formed in accordance with the presentinvention is that the nanocrystals shield the charge carriers (i.e.,holes and electrons) from defects that are present in bulk crystallinematerials. These crystallographic defects are known to limit theachievable internal quantum efficiency in devices constructed from bulkmaterials. Another advantage of the confinement is that the chargecarriers do not diffuse over a long distance, such that higher carrierconcentration levels can be formed before Auger recombination (whichdoes not result in emission of light) becomes dominant. As such,radiative recombination becomes virtually the only process by whichelectrons and holes recombine. In other words, in exemplary embodimentsof the present invention, the potential barrier formed around eachnanocrystal inhibits the diffusion of injected charge carriers, andincreases the probability that these charge carriers will recombine toemit light. Hence, in exemplary embodiments of the present invention,the internal quantum efficiency is very high.

As a result of the novel structure and function of the nanocrystal LEDsof current invention, a strong light emission is realized with quantumefficiencies (defined as the ratio of photons emitted to electron-holepairs generated or injected) approaching 100%. Moreover, this level ofefficiency is an average for clusters of silicon nanocrystals at roomtemperature. Further, the wavelength of light emitted by thesenanocrystals can be selectively changed by varying the size of thenanocrystals, from the near infrared through the entire visiblespectrum, as known to those skilled in the art.

An exemplary embodiment of a light emitting device in accordance withthe current invention is shown schematically in FIG. 1. As depicted, thedevice includes a gate contact 100, a control oxide 102, a nanocrystalfloating gate 104, and a tunnel oxide 106. The light emitting device isformed on a substrate 108. The substrate 108 can be any materialsuitable for the deposition of the necessary materials, such as, forexample, silicon. Further, the control oxide 102 and the tunnel oxide106 that separates the nanocrystal floating gate from the substrate 108can also be any suitable materials, such as, for example, silicondioxide (SiO₂).

As can be seen in FIG. 1, the layout of the light emitting devicestructure in one exemplary embodiment is based on conventional metalinsulator semiconductor/metal oxide semiconductor (MIS/MOS) floatinggate devices. The floating gate 104 includes an array of semiconductornanocrystals, and the gate contact 100 is optically semi-transparent ortransparent to enable optical access to the semiconductor nanocrystals104, such that the emitted light can be visible outside of the device.In such an embodiment the nanocrystals may be made of any suitableelemental or compound semiconductor, such as, for example, silicon,gallium arsenide or cadmium selenide. Although MIS/MOS designs arediscussed above, in other embodiments, waveguide and/or micro-resonatorintegrated design geometries may also be used for the inventive lightemitting device structure.

In the light emitting device of FIG. 1, charge carriers are sequentiallyinjected into the nanocrystals of the floating gate 104. The chargecarriers then form excitons that recombine to emit photons. The lightemitting device can be fabricated using a modified CMOS process andallows for power efficient light sources, or in devices that are scaleddown to include a single nanocrystal emitter, novel “photon on demand”sources, and local excitation sources for plasmonic circuits. The lightemitting device of FIG. 1 utilizes the specific material properties ofthe semiconductor nanocrystals embedded in the insulating matrix toyield light emission in response to an electrical pump signal. Chargeinjection of electrons and holes can be accomplished through a varietyof transport mechanisms including, but not limited to, direct andFowler-Nordheim tunneling and hot carrier injection.

The gate contact 100 should be electrically continuous such that asubstantially uniform bias can be created above the channel. When thegate bias is programmed negatively with respect to the substrate 108, asingle hole tunnels into each nanocrystal in a self-limiting process.Upon return to a zero bias condition, these holes are stored in thenanocrystals of the floating gate 104. The gate bias is then set to apositive value, drawing a single electron into each nanocrystal.Alternatively, the gate bias may switch from a negative to a positivebias, substantially instantaneously.

When an electron encounters the previously stored hole, an exciton isformed. This exciton can then recombine radiatively forming a lightemission, or non-radiatively to transfer energy to a secondary emittersuch as a rare earth ion or a suitable plasmonic structure. In anotheralternative non-radiative recombination, excitons prepared in thenanocrystals can be selectively quenched by the application of anadditional programming pulse as follows: neutral nanocrystals containingexcitons can be programmed with an additional hole or electron, leadingto the non-radiative Auger de-excitation of the exciton. In thisembodiment, the energy of the exciton is radiated away as phonons in thesupporting SiO₂ matrix (i.e., gate oxide), and the nanocrystals are leftwith a single charge that can be reused or ejected into the channel. Bymodifying the programming sequence the light emitting device may be madeto radiate only during a short window of time that can potentially bemuch less than the natural radiative lifetime. This may allow the lightemitting device to be used as a novel “photon on demand” source,especially in devices that are scaled down to include a singlenanocrystal. For example, in one embodiment of the invention a singlenanocrystal could act as an electrically pumped local “photon on demand”excitation source for plasmonic circuits.

FIGS. 2A to 2C show a schematic representation of the field-effectelectroluminescence mechanism of a light emitting device 200. It can beseen in FIGS. 2A to 2C that the array of silicon nanocrystals embeddedin the gate oxide of the light emitting device can be sequentiallycharged with electrons (via Fowler-Nordheim tunneling), and holes (viaCoulomb field-enhanced Fowler-Nordheim tunneling) to prepare excitonsthat radiatively recombine. Band diagrams 220 and 230 of FIGS. 2A and2B, respectively, depict the relevant tunneling processes for thesephenomena. Although the light emitting device 200, has substantially thesame structure as the light emitting device of FIG. 1, it should beunderstood that any of the structures discussed in the currentspecification may be operated in this manner.

In the light emitting device 200, a floating gate array of siliconnanocrystals 204 is formed from well-passivated silicon nanocrystalssmall enough to have excitonic emission energies that are higher thanthe bulk silicon emission energy. The light emitting device 200 of FIGS.2A-2C has a three-terminal transistor structure in which nanocrystalsare embedded in the gate oxide of a conventional MOSFET. Thesenanocrystals form the floating gate and may also be referred to as afloating gate array of nanocrystals. The gate contact 202 issemi-transparent or transparent at the device emission wavelength and isdesigned to provide substantially uniform control of the channelpotential.

Under appropriate bias conditions, a nanocrystal array as shown in FIGS.2A-2C may be programmed with electrons or holes. In the specific case ofa device fabricated on a p-type substrate, the electrons may beprogrammed from an inversion layer and the holes programmed from thechannel in accumulation. In one embodiment of the invention, electronsand holes are injected sequentially into the nanocrystals in the gateoxide from the channel of the MOSFET by applying an alternating electricfield to the gate. The sequential accumulation of electrons and thenholes within these nanocrystals on each cycle thereby results inrecombination and the emission of light. In this mode of operation, thecharge carriers are injected from only one side of thenanocrystal-embedded matrix. The charge carrier tunneling efficiency issubstantially governed by the distance between the nanocrystals and thechannel, and not by their density or the total matrix thickness. Sincethe fields necessary to produce light in these three-terminal devicesare much lower than for reported two-terminal based nanocrystal devices,oxide degradation due to thermal effects is reduced or minimized,thereby improving the prospects for the long-term operation of the lightemitting device.

As can be seen in FIG. 2A, the silicon nanocrystals (represented bycircles 204) form the floating gate of a transistor (e.g., MOSFET) 200with an optical thin conducting gate contact 202. The transistor 200 hasa drain 208 and a source 210, and has a substrate which is grounded, forexample. The transistor has a channel region 209 between the drain 208and the source 210. During operation, when the gate bias is programmedpositively with respect to the substrate (i.e.,V_(gate)>V_(e-injection)>V_(threshold)) using a power source 212, forexample, a single electron 206 tunnels into each nanocrystal, as can beseen in FIG. 2A. On the other hand, when the gate bias is programmednegatively with respect to the substrate, a single hole would tunnelinto each nanocrystal.

The gate bias is then set to a negative value using a power source 213with respect to the substrate, as can be seen in FIG. 2B. Then a singlehole is drawn into each nanocrystal where it encounters the previouslystored electron and forms an exciton, which can then recombineradiatively and emit light 214 as can be seen in FIG. 2C. Duringoperation as the gate bias is first set to a negative value and thenpositive, the holes would first be stored in the nanocrystals and thenelectrons would be drawn to the nanocrystals upon application of thepositive gate bias thereby forming excitons together with the holesalready in the nanocrystals.

FIGS. 3A to 3C, show a schematic representation of the formation ofexcitons and their emission as light in an exemplary embodiment of thecurrent invention. For example, these figures respectively show that aninitially neutral nanocrystal is charged with a single hole 302 when thegate voltage V_(gate)<0 (FIG. 3A); that the bias is then reversed, i.e.,V_(gate)>0 (FIG. 3B), a single electron 304 is introduced to form anexciton; and the exciton then recombines radiatively to emit light 306(FIG. 3C). On the other hand, FIGS. 4A and 4B show schematicillustrations of a non-radiative energy transferring excitonrecombination in accordance with another embodiment of the currentinvention. As can be seen, a non-radiative emission results from a hole402 and an electron 404 (FIG. 4A), with near field energy transfer to asecondary emitter 406 (FIG. 4B), and a subsequent light 408 emissionfrom the secondary emitter 406 (FIG. 4C). One of ordinary skill in theart will understand that the secondary emitter can be made of anysuitable material, such as, for example, a rare earth ion, a nanocrystalof another material, or an appropriately designed plasmonic structure.

FIGS. 5 to 8 show a variety of test data for exemplary embodiments ofthe devices in accordance with the current invention.

FIG. 5 shows a comparison of nanocrystal photoluminescence (PL) 500excited through a semi-transparent gate contact to nanocrystalelectroluminescence (EL) 502. These spectra can be attributed to theradiative recombination of excitons within silicon nanocrystals. It canbe seen in FIG. 5 that both PL and EL spectra peak near 750 nm with afull-width at half-maximum of approximately 160 nm. These emissionwavelengths are typical for silicon nanocrystals of approximately 2 to 4nm in diameter. The spectra are inhomogeneously broadened by the sizedistribution of silicon nanocrystals in the array.

The electrical excitation process can be understood in more detail byconsidering a time-resolved electroluminescence trace shown in FIG. 6.When the gate is negatively biased (600) at −6V, the p-type channel isin strong accumulation. During this time, the nanocrystal array ischarged with holes by Fowler-Nordheim tunneling across the tunnel oxideas shown in FIG. 2A. The frequency response suggests that this initialcharge-injection process occurs on a timescale of approximately 100microseconds. When the gate is positively biased above threshold (602)at +6V, an electron inversion layer is formed. Electrons are injectedinto the hole-charged nanocrystals by a Coulomb field-enhancedFowler-Nordheim tunneling process as shown in FIG. 2B. By this processquantum-confined excitons are formed.

Using Wentzel-Kramer-Brillouin (WKB) approximation-based analysis it hasbeen estimated that a previously injected hole can dramatically enhancethe electron tunneling rate of the Fowler-Nordheim rate for electrontunneling into a neutral nanocrystal. The onset of electroluminescence(606) is well fit by a single exponential rise (τ˜2.5 microseconds) atthe applied +6 V gate bias, suggesting that electron injection isenhanced by a factor of approximately 40 by the presence of holes in thenanocrystal array. The measured electron-tunneling rate enhancement isconsistent with Coulomb field-enhanced Fowler-Nordheim tunneling througha tunnel oxide thickness of approximately 4 nm, which is theexperimental tunnel oxide thickness targeted in one exemplaryfabrication process it should be understood that by reducing thethickness of the tunneling oxide, faster tunneling rate can be achieved.For example, switching speeds of 1 GHz may be realized by the carefulselection of the nanocrystal material and the thickness of the tunneloxide.

Although not to be bound by theory, the observation ofelectroluminescence implies that holes already confined in thenanocrystals have emission times for tunneling back to the channel thatexceed the Coulomb field-enhanced Fowler-Nordheim tunneling time forelectron injection from the inversion layer. The observation ofelectroluminescence also substantially precludes the injection ofmultiple electrons into the hole-charged nanocrystals as radiativerecombination of excitons is evidently not quenched by Augerrecombination, which is known to be an efficient non-radiativerecombination mechanism in silicon nanocrystals containing an excitonand an addition charge of either polarity.

The emission shown in FIG. 6 decays (608) from its peak value as thepreviously injected holes are consumed by electrons in exciton formationand decay. A stretched exponential equation with a time constant ofapproximately 30 microseconds (ideality factor ˜0.5) can be used tocharacterize the observed decay. The time constant is longer than thephotoluminescence decay lifetime observed under optical excitation at anapplied gate bias of +6V (τ˜5 microseconds, ideality factor ˜0.7). Thelonger electroluminescence-decay time constant may reflect an absence ofnon-radiative recombination paths that are present for some fraction ofthe excitons recombining under illumination. Also, an indirect chargingprocess involving inter-nanocrystal carrier migration could increase thetime for exciton formation.

When electroluminescence is no longer observed (610), there are no morequantum-confined holes left in the array to form excitons. Electronscontinue to tunnel into the nanocrystal array due to the positive gatebias, resulting in each nanocrystal becoming recharged with an electron.Multiple charging of nanocrystals is substantially suppressed byCoulombic field-inhibition of Fowler-Nordheim tunneling into chargednanocrystals. The tunneling rate of second electrons may be suppressedby a factor of approximately 300 over the Fowler-Nordheim rate fortunneling of the first electron into a neutral nanocrystal.

These programmed electrons can now form excitons when the gate voltageis switched to a negative potential (604) sufficient (e.g., −6.0V) toenable hole injection from the accumulation layer. This process ischaracterized by a faster single-exponential rise (612) inelectroluminescence (τ˜240 nanoseconds) and a faster stretchedexponential decay (τ˜10 microseconds, ideality factor ˜0.5) (614).

As can be seen in FIG. 6, the electroluminescence peak associated withhole injection into electron-charged nanocrystals is smaller inmagnitude and shorter in duration than the electroluminescence peakassociated with electron injection into hole-charged nanocrystals. Thisasymmetry may be ascribed to stored electron loss by tunneling back tothe channel during hole injection at positive gate bias. This lossmechanism is more apparent for hole injection into electron-chargednanocrystals due to the smaller conduction-band offset (˜3.2 eV) thanvalence-band offset (˜4.7 eV) between silicon and silicon oxide.

From a study of the data in FIG. 6 it can be seen thatelectroluminescence is clearly correlated with injection of the secondcarrier, indicating that field-effect-induced electroluminescence is dueto programmed exciton formation rather than impact excitation resultingfrom a DC leakage current through the gate stack. The lack of emissionunder DC electrical bias is further confirmed by an examination of thefrequency dependence of electroluminescence in FIG. 7. For a constanttwo-second measurement integration time electroluminescence is initiallyobserved to increase linearly with increasing driving frequency becauselight is collected from an increasing number of integrated completedcycles.

It can also be seen in FIG. 7 that electroluminescence emission peaks ata frequency of 10 kHz, and then begins to decrease, which can beattributed to several effects. As the driving frequency is increased,the number of excitons formed at positive-to-negative ornegative-to-positive bias transitions begins to decrease due toincomplete initial electron (hole) charging. From the 10 kHz peak in thefrequency response, the charge injection into neutral nanocrystals mayrequire approximately 100 microseconds. At frequencies aboveapproximately 30 kHz, the pulse duration becomes shorter than theradiative lifetime of silicon nanocrystals and some fraction of theexcitons will not recombine. At even higher frequencies, the emissionmay be further limited by the gate capacitive charging time constant.

As shown in FIG. 8, electroluminescence increases dramatically withincreasing root mean squared (r.m.s.) drive voltage, and has asaturation onset at approximately 4 V_(rms) The tunnel oxide field isproportional to gate voltage and the electroluminescence intensity isproportional to the tunneling current. Thus, an equivalentFowler-Nordheim plot that is well fit by a linear relation can beconstructed. This observation is consistent with initial electron andhole injection into neutral nanocrystals being dominated by aFowler-Nordheim tunneling process. A distribution of tunneling oxidebarrier thickness should be present due to the implantation-basednanocrystal formation process. The slope of the Fowler-Nordheim plot istherefore determined by the average value of the effective fieldstrength, which will vary across the array of nanocrystals.

Although specific silicon devices have been discussed thus far,field-effect electroluminescence is a general carrier-injection methodin which electrons and holes are sequentially injected into ananocrystal array to form excitons. Accordingly, silicon nanocrystalsand silicon dioxide tunnel barriers might be replaced by otherexciton-confining nanostructures or injection-barrier materials. Devicesdesigned to operate by field-effect electroluminescence may be termedfield-effect light-emitting devices (FELEDs) as the principle ofoperation differs significantly from that of pn-junction LEDs, and theresulting device structures rather resemble field-effect transistorsinstead.

FELEDs may also be more robust than LED-based nanocrystal light sourcesbecause the carrier-injection process can be controlled. In previouslyreported nanocrystal devices, the optical centers are excited by impactionization processes in which hot carrier energy creates ananocrystal-bound exciton. In such a process, excess hot carrier energyis lost to thermalization. Over time, excitation by impact ionizationdegrades the dielectric quality and can lead to oxide wear-out anddevice failure. Field-effect-induced carrier injection may enable theproblems associate with impact ionization excitation to be circumventedthrough controlled Fowler-Nordheim tunneling.

During testing of the light emitting device, electroluminescence hasbeen observed at an energy (Eopt) of approximately 1.65 eV, and atdriving gate voltages as low as 2.5V. However, it may be appropriate toassume that all nanocrystals in an ideal device can be pumped at thislow gate voltage. By requiring continuity of the electric displacementthrough a gate stack including 8 nm control oxide, 3 nm diameternanocrystals, and 4 nm tunnel oxide; and neglecting depletion, apotential drop of approximately 0.77 V from the channel to thenanocrystal array can be found. This potential energy drop may beentirely lost to inelastic carrier scattering during Fowler-Nordheimtunneling of the carrier into the nanocrystal.

In order to drive this injection current, the gate capacitance shouldpreferably be charged. A parallel plate model suggests thatapproximately 0.6 eV is required to cycle the gate above one nanocrystalfrom 2.5V to −2.5V. A self-capacitance energy cost for chargeconfinement in a nanocrystal of approximately 0.12 eV self-capacitanceenergy cost may be further included. In this case, the nanocrystal ismodeled as a 3 nm diameter sphere in a silicon dioxide matrix. If othersources of loss are neglected, an energy (Eel) of approximately 3 eV maybe required to program one exciton, in contrast to approximately 1.1 eVrequired for exciton formation in an ideal silicon LED.

It should be noted for the silicon FELED that the internal quantumefficiency of a well-passivated silicon nanocrystal can be very high. Ifnon-radiative excitonic recombination could be completely suppressed inthe nanocrystals, the silicon FELED could conceivably have an internalpower efficiency (ipe) as high as (ipe=Eopt/Eel) approximately 55%. Foran ideal SiO₂ matrix silicon FELED fabricated on an optically absorbingsubstrate, external power efficiency (epe) is bounded to approximately10%, based on consideration of the oxide index of refraction (n=1.46).By integrating an appropriate back reflector into a FELED devicefabricated with a thin (e.g., silicon-on-insulator) substrate, the idealcase external power efficiency could approach 20%.

The light emitting device according to exemplary embodiments of thepresent invention can be readily fabricated using a standard CMOScompatible process, for example. The structure can therefore beintegrated as a light source in conventional silicon based integratedcircuits. The floating gate in the semiconductor nanocrystal lightemitting devices can also be formed using the following methods. Each ofthe following methods are known to those skilled in the art. Further,any other suitable methods can be used to form the nanocrystals.

First, the nanocrystals can be synthesized through implantation andannealing where a low energy silicon ion is implanted into the gateoxide, followed by annealing to nucleate and grow silicon nanocrystals.The depth at which silicon nanocrystals form can be changed by varyingthe implantation energy. The implantation depth effectively determinesthe gate oxide thickness. The silicon nanocrystal size and density canbe modified by varying the post-implantation annealing conditions or bychanging the silicon implantation dose. Due to the stochastic nature ofthe implantation process as well as the nucleation and growth process,the silicon nanocrystals may show some size dispersion.

The nanocrystals may also be formed using aerosol fabrication,self-assembly fabrication, or conventional chemical vapor deposition(CVD) techniques. In aerosol fabrication, size-selected siliconnanocrystals formed by an aerosol method are deposited. This techniquepotentially offers superior size selection and nanocrystal passivationoptions. In the self-assembly fabrication, some material systems allowfor self-assembly of nanocrystals in solution (solid, liquid, orgaseous). Such techniques present another possible fabrication pathway.

In a nanocrystal or a quantum dot, the color of the light emission isdetermined by the band gap of the material, and in a bulk material theband gap is a fundamental property of the material. When the material ismade very small, such as a nanocrystal or a quantum dot, the band gapenergy is increased due to quantum mechanical effects. In other words,size-dependent band gap is achieved. Hence, it is possible to, withincertain material constrains, change the band gap simply by changing thesize of the nanocrystals. By way of example, when the nanocrystals havea diameter of 2.5 nanometers or less, light emission closer to the redor even down to yellow may be achieved. Accordingly, by controlling thesize of the nanocrystals, light emitting devices having red, green andblue colors may be fabricated, which can be used in a full color displaydevice.

In addition a number of semiconductor materials may be used includingother semiconductor materials, such as, for example, cadmium selenide(CdSe) nanocrystals can be used. There are a number of vendors whosupply CdSe nanocrystals in colors throughout the visible range of thespectrum.

The transparent gate contact for optoelectronic geometries should beconductive and have a sufficient charge density to apply a well-definedfield across the silicon nanocrystal doped layer. Some possible gatematerials for the gate contact are listed below. Of course, it should beunderstood that any other suitable material can be used as well to formthe transparent gate contact.

First, thin film polycrystalline silicon can be used. Because of thesignificant optical absorption of silicon at wavelengths shorter than1.1 microns, the thickness of the polysilicon gate contact should besufficiently thin to allow transmission of the emitted wavelength.Alternatively indium tin oxide (ITO) may be used as this material is atransparent conductor commonly used in photocell applications.

Alternatively, thin metal films can be used. As in the case ofpolysilicon, the thickness must be optimized for sufficienttransmission. In such an embodiment, free carrier absorption may besignificant. Film stability may necessitate the development ofappropriate wetting layers for particular barrier materials systems.Further, patterned contacts may be used. By way of example, atransparent or partially transparent gate could be formed by fabricatinga patterned gate contact in which a significant fraction of the gatearea is open, i.e., not covered by the gate contact material. Thisdesign should be optimized to allow for efficient charge injection.

While the extensive understanding of silicon nanocrystals in SiO₂/Si MOSstructures make this type of device an obvious choice for a modelsystem, the relatively long (typically microseconds) radiative excitonicrecombination lifetimes in silicon may limit the luminosity of a siliconbased device. By way of example, as a light source, perhaps hundreds oflumens per square centimeter of device area may be provided by the lightemitting device of the present invention. For example, assuming 10¹³nanocrystals per cm² of nanocrystal density, 20% external quantumefficiency, 2 eV (red) photons and a device cycle rate of 1 MHz, 435lumens/cm² may be realized from 3.2 Watts of optical power (considering680 lumens/Watt) in an ideal case.

Direct band gap semiconductor nanocrystal arrays may provide greaterluminosity due to a shorter excitonic radiative decay lifetime. The useof nanocrystals with a band gap less than 1.1 eV (such as lead selenidenanocrystals) can be optically accessed through silicon, which istransparent at these energies, allowing for integration of infraredlight sources with silicon optical waveguides. Such materials systemsalso offer the promise of integration with telecommunications systemsoperating in the infrared.

While certain exemplary embodiments have been described above in detailand shown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative of and not restrictive of the broadinvention. It will thus be recognized that various modifications may bemade to the illustrated and other embodiments of the invention describedabove, without departing from the broad inventive scope thereof. In viewof the above it will be understood that the invention is not limited tothe particular embodiments or arrangements disclosed, but is ratherintended to cover any changes, adaptations or modifications which arewithin the scope and spirit of the invention as defined by the appendedclaims, and equivalents thereof.

1. A light emitting device including a transistor structure disposed ona semiconductor substrate, the transistor structure having a sourceregion, a drain region, a channel region between the source and drainregions, and a gate oxide on the channel region, the light emittingdevice comprising: at least one nanocrystal embedded in the gate oxide;a gate contact made of semitransparent or transparent material, andformed on the gate oxide; and at least one charge injection sourceapplied between the gate oxide and the semiconductor substrate andprogrammed such that the at least one nanocrystal is initially injectedwith at least one first type charge carrier followed by at least onesecond type charge carrier to form excitons used to produce a lightemission.
 2. The light emitting device of claim 1, wherein a color ofthe emitted light is varied by varying the size of the nanocrystal. 3.The light emitting device of claim 1, wherein a single nanocrystal isembedded in the gate oxide.
 4. The light emitting device of claim 1,wherein a plurality of nanocrystals are embedded in the gate oxide. 5.The light emitting device of claim 1, wherein the at least onenanocrystal comprises an array of nanocrystals.
 6. The light emittingdevice of claim 5, wherein the array of nanocrystals form a floatinggate of nanocrystals.
 7. The light emitting device of claim 1, whereinone of the first type charge carriers is injected into the at least onenanocrystal, and one of the second type charge carriers is injected intothe at least one nanocrystal.
 8. The light emitting device of claim 1,wherein the excitons recombine to emit light.
 9. The light emittingdevice of claim 1, wherein a de-excitation of the excitons results inthe energy of the excitons being transferred to excite a secondaryemitter of light.
 10. The light emitting device of claim 9, wherein theenergy transfer is accomplished through a non-radiative de-excitationprocess selected from the group consisting of near field energytransfer, resonant energy transfer, or carrier transfer.
 11. The lightemitting device of claim 9, wherein the secondary emitter of light isselected from the group consisting of an elemental or compoundsemiconductor nanocrystal, a molecular light emitter, or a metallicplasmonic structure.
 12. The light emitting device of claim 11, whereinthe semiconductor nanocrystal is made of a material selected from thegroup consisting of gallium arsenide, lead selenide, or cadmiumselenide.
 13. The light emitting device of claim 1, wherein the firsttype charge carriers are electrons and the second type charge carriersare holes.
 14. The light emitting device of claim 10, wherein thenon-radiative de-excitation of the excitons is deliberately encouragedto suppress the emission of light.
 15. The light emitting device ofclaim 9, wherein the first type charge carriers are holes and the secondtype charge carriers are electrons.
 16. The light emitting device ofclaim 1, wherein the bias source sequentially injects the first andsecond type charge carriers into the nanocrystals through the channelregion of the transistor structure.
 17. The light emitting device ofclaim 1, wherein the first type charge carriers are provided to thenanocrystals when a first bias is applied between the gate oxide and thesemiconductor substrate, and the second type charge carriers areprovided to the nanocrystals when a second bias is applied between thegate oxide and the semiconductor substrate.
 18. The light emittingdevice of claim 17 wherein the first and second biases have inversepolarities of each other.
 19. The light emitting device of claim 1,wherein the nanocrystals are made of one of either an elemental orcompound semiconductor.
 20. The light emitting device of claim 1,wherein the nanocrystals are made of a material selected from the groupconsisting of silicon, gallium arsenide, lead selenide, or cadmiumselenide.
 21. The light emitting device of claim 1, wherein the gatecontact comprises a material selected from the group consisting of thinfilm polycrystalline silicon, indium tin oxide (ITO), a thin metal film,or a patterned contact.
 22. The light emitting device of claim 1,wherein the transistor structure is fabricated using a CMOS compatibleprocess.
 23. The light emitting device of claim 1, wherein thetransistor structure has a structure of a metal-oxide-semiconductorfield-effect transistor (MOSFET).
 24. The light emitting device of claim1, wherein the first and second type charge carriers are injected intothe nanocrystals by tunneling through a portion of the gate oxidebetween the channel region and the nanocrystals.
 25. The light emittingdevice of claim 24, wherein the first type charge carriers are injectedby Fowler-Nordheim tunneling, and the second type charge carriers areinjected via Coulomb field-enhanced Fowler-Nordheim tunneling.
 26. Amethod of emitting light from a light emitting device including atransistor structure formed on a semiconductor substrate and having asource region, a drain region, a channel region between the source anddrain regions, a gate oxide on the channel region, and a plurality ofnanocrystals embedded in the gate oxide, the method comprising:injecting first type charge carriers into the nanocrystals; and afterinjecting the first type charge carriers, injecting second type chargecarriers into the nanocrystals, such that the first and second typecharge carriers form excitons used to emit the light.
 27. The method ofclaim 26, wherein injecting first type charge carriers comprisesinjecting one of the first type charge carriers into each of thenanocrystals, and injecting second type charge carriers comprisesinjecting one of the second type charge carriers into each of thenanocrystals.
 28. The method of claim 26, wherein a de-excitation of theexcitons results in the transfer of the energy of the excitons to asecondary emitter of light.
 29. The method of claim 26, wherein theenergy transfer is accomplished by a non-radiative de-excitation processselected from the group consisting of near field energy transfer,resonant energy transfer, or carrier transfer.
 30. The method of claim29, wherein the non-radiative de-excitation is deliberately encouragedto suppress the emission of light.
 31. The method of claim 26, whereinthe first and second type charge carriers are sequentially injected intothe nanocrystals through the channel region of the transistor structure.32. The method of claim 26, wherein injecting first type charge carrierscomprises applying a first bias between the gate oxide and thesemiconductor substrate, and wherein injecting the second type chargecarriers comprises applying a second bias between the gate oxide and thesemiconductor substrate.
 33. The method of claim 32, wherein the firstand second biases have inverse polarities of each other.
 34. A lightemitting device comprising: a gate oxide disposed on a semiconductorsubstrate; a floating gate array of nanocrystals embedded in the gateoxide; a transparent or semi-transparent gate contact formed on the gateoxide; and at least one charge injection source applied light emittingdevice and programmed such that the nanocrystals are initially injectedwith at least one first type charge carrier followed by at least onesecond type charge carrier to form excitons used to produce a lightemission.